So, why the buildup?

She was storing her horn with the case flat on the floor, and the Bb side down. And, she did not empty the water from the slides before putting the horn away. The water would collect at the lowest point, which was inside the valves on the Bb side, it would evaporate, and the solids that were suspended in that solution remained as hardened deposits.

So, why were there no deposits on the F side of the horn?

Those channels are nice and clean. There are two reasons. First, she put oil down the F side, and never down the Bb side. That oil protected the metal from the water and deposits. And second, she used her F side more, and the wrap of her horn allowed the water to drain away more readily while she played.

Did the player tell me this?

No, the horn did while it was in the shop for a cleaning. All I had to do was confirm it when she picked up the horn. We discussed how she could change her cleaning and storing regimen to lengthen the time between cleanings and keep her horn in good working order.